The present invention relates to an apparatus for controlling a hydraulic pressure used for speed change performed by an automatic transmission of an automobile, and particularly to a hydraulic control apparatus for an automatic transmission which is intended to directly control a hydraulic pressure to be supplied to a clutch on the basis of an electric signal.
The control apparatus of this type has been known, for example, from Nissan's maintenance guide titled "Full Range Electronically Controlled Automatic Transmission" issued by NISSAN MOTOR on March, 1987. In this control apparatus, an accumulator is disposed between a manual spool valve and a forward clutch and another accumulator is disposed between the above spool valve and a reverse clutch. Another known hydraulic control apparatus generates a variation in torque of an output shaft of an automatic transmission when the position of a shift lever of an automobile is changed between an N (Neutral) range and a D (Drive: forward) range or between the N range, and an R (Reverse) range is relaxed by mechanically and electrically controlling a line pressure introduced in the above spool valve. At the present time, the hydraulic control apparatus having such a structure has been extensively used for automobiles.
The above-described prior art hydraulic control apparatus, however, is disadvantageous in that provision of two accumulators for forward and reverse clutches makes larger the size of a transmission using the control apparatus, and that the accuracy of control for suppressing a variation in torque is deteriorated due to changes in mechanical components with time.